1. Technical Field
The present invention relates to a data communication apparatus and method, and more particularly, to an integrated card for a multimedia personal computer capable of concurrently performing a network communication function and an image overlay function for a video display monitor.
2. Background Art
Data communication through a communication network is perhaps one of the most important applications commonly performed on a multimedia personal computer. The asynchronous transfer mode (ATM) was selected by the International Telegraph and Telephone Consultative Committee (CCITT), now referred to as the International Telecommunications Union (ITU), as the preferred underlying transport technology within Integrated Service Digital Networks Integrated Service Digital Networks (ISDN) and Broadband ISDN (B-ISDN) for carriage of a complete range of user traffic. Under the standard asynchronous transfer mode video, audio and text data are all broken up and transmitted in a sequence of fixed length packets, each referred to as a "cell" including 53 bytes. The transmission may be performed according to a variable bit-rate, with burst characteristics for video and audio signals, or at constant bit-rate for data information files. Video and audio (e.g. voice) signals are generally more sensitive to time delay than to loss, whereas some data signals do not require real-time transmission, but may be highly sensitive to loss.
Currently, there are products available, such as those disclosed in U.S. Pat. No. 4,954,970 for Video Overlay Image Processing Apparatus issued to Walker et al., U.S. Pat. No. 5,271,097 for Method And System For Controlling The Presentation of Nested Overlays Utilizing Image Area Mixing Attributes issued to Barker et al., and U.S. Pat. No 5,283,867 for Digital Image Overlay System And Method issued to Bayley et al., which allow a multimedia personal computer to process high quality audio, text, graphics, animation and special effects data received from either an external communication network or the computer central processing unit for video display. Conventionally however, the network communication card and the video overlay card are constructed and installed separately and must therefore transfer information over the computer bus.
High-resolution motion picture data having a relatively rapid frame rate (e.g. 15-30 frames per second), sometimes accompanied by audio data, are typically compressed according to a compression technique designated MPEG-1, the first standard codex algorithm recommended by the Motion Picture Expert Group, a committee of Working Group 8 in the International Standards Organization (ISO), although similar H.261 or later protocols recommended the CCITT (ITU) or other organizations are occasionally used. The well known MPEG-1 compression technique is able to convert video data captured at up to 27 MB per second to more manageable data rates for transfer and storage, by performing both intraframe and interframe compression. Intraframe compression refers to a plurality of techniques performed on each frame, beginning with image scaling, luminance and chrominance separation, and color subsampling to discard most of the chrominance information. Each 8.times.8 pixel block is then encoded by a Discrete Cosign Transform (DCT) into a set of numbers describing a level of image detail, and the DCT numbers are divided by a constant and rounded off in a quantitizion step. Finally, each string of identical quantitized numbers is run-length encoded to assign a small token indicating simply the value and the number of times it repeats, then the tokens are converted into to variable length symbols called Huffman codes. Further compression is achieved with interframe techniques using motion prediction and motion detection to eliminate redundant blocks appearing in more than one frame. MPEG decompression boards, such as Sigma Designs' RealMagic card, decode compressed motion picture data at up to 30 frames per second (fps) according to overall system capabilities, and use an overlay processor to punch a hole in a simultaneously displayed graphics image in order to play the decoded motion pictures within an empty Window on the video monitor. The overlay image is sometimes provided directly to a separate graphics card through a feature connecter, but the graphics image processing circuitry may also be incorporated onto the same card as the decompression and image overlay processor. Higher compression ratios reduce the bandwidth requirements, but generate a corresponding increase in image artifacts produced by data lost during compression.
Compressed image and audio data, encoded according to a designated compression codex, such as MPEG-1, electronic mail data and other text data received through a dedicated network communication card, are conventionally transferred via the well known 8-bit Industry Standard Architecture (ISA) bus to a computer storage medium (e.g., a hard disk), then the MPEG compressed data are supplied to a decompression and overlay processor via the same ISA bus. The aforementioned conventional technique for sequentially performing communication, decompression, and image overlay functions often places a heavy operating burden on the central processing unit (CPU), the computer storage medium, and the ISA bus. As a result, overall system performance can deteriorate, and the ISA bus may be unavailable for use by other devices during the communication process. Even a faster wide computer bus, such as the 16-bit Extended Industry Standard Architecture (EISA) bus, capable of up to 4 MB per second, and the competing 32-bit local bus standards set forth by VESA and PCI, are unable to address the operating burden placed on the central processing unit and computer storage medium by the conventional data communication techniques. Accordingly, while communication network cards and decompression and overlay cards are known in the art, they have not previously been combined into an integrated device capable of effective responding to the increased demands imposed by processing of compressed data on a multimedia computer system.